<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <script>
        const PI=3.1415926
        let r=2
        document.writeln("圆周率：" + PI + "面积：" + 2*PI*r**2)
        var a = 1, b = 2, c
        c = a
        a = b
        b = c
        console.log(a,b)
    </script>
</body>
</html>